This application relates to tower grain dryers, and, in particular to a tower dryer with a heat reclamation/recovery system and, optionally, a counter-flow cooling section.
Currently available grain tower dryers 10, such as shown schematically in FIG. 1, typically comprise a central plenum 12 defined by a porous (or air pervious) wall 14. The tower also includes an outer porous (or air pervious) wall 16 spaced from the plenum wall 14. The plenum wall and outer wall define an annular column 18 through which grain to be dried flows. A heater/blower 20 is typically positioned within the plenum. The heater/blower draws cool air in through a bottom portion of the grain dryer and across the grain column, as shown by the arrows A1 in FIG. 1, which cools the heated grain and simultaneously preheats the air which cooled the grain prior to entering the heater/blower. The preheated air enters the heater/blower to be heated to the drying temperature and is expelled out through an upper portion of the grain column, as shown by the arrows A2. Hence, the grain falls through an upper heating section (where the grain is dried) into a lower cooling section (where the grain is cooled).
Dryers of the tower type can process anywhere from 1000 bushels per hour to over 10,000 bushels per hour. It can be appreciated that any improvement in drying efficiency can have a significant benefit. In addition dryers of this type, with their very high drying rates and cross-flow cooling/heat reclamation, are often cited as causing some damage to the drying grain due to the rapid cooling of hot grain with very cool air. Thus, improvements to the cooling section can result in improved grain quality as well as improved heat reclamation.
As can be appreciated, in a dryer with a cross-flow of cooling air, such as shown in FIG. 1, the hot dried grain which exits the drying section will encounter air substantially at ambient temperature. This contact of the hot grain with substantially cooler air can cause high moisture corn to crack and break.
Additionally, high moisture grain, such as high moisture corn, will be subjected to more heat during the drying process than is typically required under normal harvest conditions. This can make the grain more susceptible to breakage. Broken corn increases the amount of fine material (“grain fines”) in storage vessels and is often more susceptible to mold while in storage. Research has shown that rapid transition from hot to cool can exacerbate breakage potential. Wet harvest conditions can also lead to the presence of other foreign materials with the grain. The Grain Inspection, Packers and Stockyards Administration (GIPSA) of the United States Department of Agriculture (USDA) defines foreign materials as: “All matter that passes readily through a 6/64″ round-hole sieve and all matter other than corn that remains on top of the 12/64″ round-hole sieve according to procedures prescribed in FGIS instructions.” The maximum allowable amount of broken corn and foreign material (BCFM) for No. 2 yellow corn is 3%. Ethanol plants will typically discount for BCFM greater than 3-4%, and corn with levels greater than 6-7% is typically subject to rejection. Facilities that may be handling atypical levels of BCFM will likely need to enhance dust control efforts and pay particular attention to dust-related safety measures. Further, abnormal BCFM can greatly affect fermentation by introducing additional unfermentable materials to the process.
It would thus also be desirable to provide a tower dryer which will reduce BCFM in corn, and reduce damage to grain in general, by providing a more gradual cooling process in the cooling section.